A. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same, and particularly relates to a large-sized and high-capacity semiconductor device with improved reliability in a joint portion between a circuit pattern on an insulating substrate and an external connection terminal, and a method for manufacturing the same.
B. Description of the Related Art
A semiconductor device for use in electric power conversion has an insulating substrate in which circuit patterns made of copper or the like are, for example, formed in an upper surface of a plate-like insulating member made of ceramic, and back-surface electrodes of semiconductor elements are bonded to the circuit patterns by soldering. In the configuration of such a semiconductor device, external connection terminals are placed to be separate from the insulating substrate. The external connection terminals are connected to the circuit patterns by wire bonding. In addition, copper foil formed on the back surface of the insulating substrate is connected to a copper base plate by soldering so that the insulating substrate can be fixed to the base plate.
Large-scale power conversion systems are requested to save space. To this end, semiconductor devices must be made large in scale and high in capacity to reduce the number of semiconductor devices used per unit power. In the configuration of a high-capacity semiconductor device where external connection terminals are connected to circuit patterns by wire bonding, a large number of wires must be used in order to secure a required current capacity. Thus, a method for bonding the external connection terminals directly to the circuit patterns is used.
The external connection terminals and the circuit patterns are often bonded by soldering. However, according to the bonding by soldering, it has been difficult to secure reliability in the bonding portions because the semiconductor device is made larger in scale and higher in capacity. That is, a base plate is deformed due to heat generated when the semiconductor is in use. As the semiconductor device increases in scale, stress on the bonding portions of the external connection terminals becomes very high due to the deformation. In addition, bonding by soldering requires heating to fuse solder. Due to the heating, the base plate may be deformed. When a plurality of bonding portions are formed integrally with the external connection terminals, the distance between each external connection terminal and each circuit pattern may be changed in accordance with bonding positions. In addition, the electric resistance of the solder is about one order of magnitude greater than that of the external connection terminals or the circuit patterns. If the semiconductor device is made higher in capacity, the amount of Joule heat generated in the solder will increase and that will increase loss when the power conversion system is in operation.
There is known a large-scale high-capacity semiconductor device in which external connection terminals are bonded to circuit patterns by ultrasonic welding in place of solder bonding (for example, see US Patent No. 2008/0211091). The ultrasonic welding is performed by applying horizontal ultrasonic vibration to a bonding end portion of each external connection terminal while applying a vertical load to a bonding interface between the external connection terminal and the circuit pattern. Ultrasonic welding can be performed at a normal temperature, in a short time and in the atmosphere, and high reliability in bonding portions can be secured due to solid state bonding between metals of one and the same material.
When a semiconductor device has a high capacity, each external connection terminal must be thickened to increase its sectional area. For example, when the external connection terminal uses a copper plate 0.8 mm thick, it is known that the reliability in the ultrasonic welding portion cannot be ensured if the Vickers hardness of the external connection terminal is higher than 80 (see Japanese Patent No. 3524360).
However, when low-hardness external connection terminals are used, the strength of the semiconductor device as a whole including the external connection terminals cannot be ensured. Particularly when the external connection terminals are further thickened to increase the capacity, a problem with respect to the long-term reliability of the semiconductor device arises, and it deteriorates due to the insufficient strength.
In addition, due to increases in the scale of the semiconductor device, external connection terminals with bonding end portions formed integrally with long bars to protrude thereon may be ultrasonically bonded with circuit patterns formed in predetermined positions on an insulating substrate. In this case, due to the processing accuracy of the external connection terminals or the assembling accuracy in bonding the external connection terminals, it is difficult to align the bonding positions thereof. Thus, there arises a problem that the bonding strength deteriorates due to the misalignment of the bonding positions to lower the long-term reliability of the semiconductor device.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.